This invention relates to ore sorting, and finds particular although not exclusive application in the sorting of gold-bearing reef from waste material. In manual sorting of gold-bearing ore, a distinction can be made between reef and waste, because the reef has a pebbly, non-homogeneous appearance, whereas the waste is generally homogeneous and non-pebbly in appearance. The pebbly appearance of reef arises because of the presence of quartz pebbles in the host matrix.
Conventional photometric sorting systems have no problem in distinguishing light areas from dark areas of a specimen rock. Such systems are therefore able to sort dark rocks from light rocks. Gold-bearing reef can be either dark or light in appearance, as can the waste.
If darkness is used as the sorting criterion for separating dark reef, dark waste reports together with the dark reef in the accept fraction. The accept fraction is therefore diluted with dark waste. Similarly, light waste and light reef report with the reject fraction.
Where a specimen is scanned and its acceptability determined in accordance with the presence or otherwise of light areas in a darker matrix (intended to sort specimens which have quartz pebbles in the matrix), the problem exists that dark waste may be bruised (e.g. as a result of collisions etc.), with the result that the apparatus detects the lighter bruised areas in the same way as it detects lighter areas in a dark reef specimen.
A system which could additionally distinguish rock specimens from one another according to whether they are pebbly or not, for example, that is in the case of good reef sorting, according to whether pebbles of quartz are present in the matrix or not, would therefore be most desirable, approximating perhaps more closely in its decisions to human decisions based on the pebbly nature or otherwise of a specimen which is sorted manually.
In the specification "ore objects" or "objects of ore" are to be regarded as objects possessing a certain physical characteristic, whereas "objects" or "waste" may possess the same physical characteristic but to a lesser degree, or may not possess the physical characteristic at all. Thus, where mention is made of sorting "ore objects" it is intended that sorting is carried out to distinguich objects possessing a certain physical characteristic from objects which either do not possess the physical characteristic or possess it to a lesser degree the physical characteristic which is to be determined in order to distinguish between objects of ore and other objects relating to the incoroporation, in objects of ore, of one or more translucent inclusions capable of re-emitting internal reflections.